Screening a black-surround color cathode-ray tube

ABSTRACT

THE SCREEN IS COVERED WITH SENSITIZED PVA AND EXPOSED FROM EACH OF THREE POSITIONS SIMULATING THE THREE BEAMS OF THE TUBE IN PROCESS BUT WITH THE EXPOSURE POSITIONS DISPLACED INWARD TOWARD THE TUBE AXIS IN THE PLANE OF DEFLECTION. THE LATENT IMAGE THUS PRODUCED ON THE SCREEN IS THEN DEVELOPED AND LIGHT-ABSORBING MATERIAL IS DEPOSITED IN THE PORTIONS OF THE SCREEN FROM WHICH UNEXPOSED PVA HAS BEEN REMOVED IN THE DEVELOPING STEP. THE SCREEN IS THEN STRIPPED OF THE REMNANT PVA LAYER AND AGAIN COATED WITH PVA. THEREAFTER, A SIMILAR EXPOSURE IS UNDERTAKEN BUT NOW WITH THE EXPOSURE POSITIONS DISPLACED OUTWARDLY FROM THE TUBE AXIS IN THE PLANE OF DEFLECTION. FOLLOWING THE THIS THERE IS A FURTHER DEVELOPMENT STEP AND AGAIN THE DEPOSITION OF LIGHT-ABSORBING MATERIAL ON THE PORTIONS OF THE SCREEN FROM WHICH PVA HAS BEEN REMOVED. THE THREE EXPOSURE POSITIONS IN EACH OF THESE TWO SEQUENCES OF PROCESS STEPS CAUSE THE MATRIX, WHICH IS MADE IN TWO SEGMENTS, TO EXHIBIT APPROXIMATELY CIRCULAR HOLES PROPERLY LOCATED TO RECEIVE PHOSPHOR AND FORM THE CUSTOMARY PHOSPHOR DOT TRIADS ON THE SCREEN OF THE TUBE.

July 18, 1972 s. KAPLAN 3,677,758

SCREENING A BLACK-SURROUND COLOR CATHODE-RAY TUBE Filed D80. 21, 1970 3 Sheets-Sheet l FIG 1 L l f 1 I u v *3 80 m H .flKop lon BL]; Atorney S. H. KAPLAN July 18, 1972 SCREENING A BLACK'SURROUND COLOR CATHODE-RAY TUBE 3 Sheets-Sheet 2 Filed Dec. 21 1970 Inventor Sam H. Koptan Attorney 00 01 w o wn m '00 oo o 4 1 O 0 HWMO July 18, 1972 s. H. KAPLAN 3,677,753

scasnume A amcmsu'rmouun COLOR cATnoDE-RAY TUBE Filed Dec. 21, 1970 3 Sheets-Sheet 5 FIG. 58

Inventor Scam H. Koplan By%M Attorney United States Patent O Int. Cl. G03c 5/00 US. C]. 96-36.! Claims ABSTRACT OF THE DISCLOSURE The screen is covered with sensitized PVA and exposed from each of three positions simulating the three beams of the tube in process but with the exposure positions displaced inward toward the tube axis in the plane of deflection. The latent image thus produced on the screen is then developed and light-absorbing material is deposited in the portions of the screen from which une'xposed PVA has been removed in the developing step. The screen is then stripped of the remnant PVA layer and again coated with PVA. Thereafter, a similar exposure is undertaken but now with the exposure positions displaced outwardly from the tube axis in the plane of deflection. Following the this there is a further develop ment step and again the deposition of light-absorbing material on the portions of the screen from which PVA BACKGROUND OF THE INVENTION The present invention is addressed to the process of making a black-surround tube of the type disclosed and claimed in Pat. 3,146,368-Fiore et al. issued on Aug. 25, 1964 and Pat. 3,36S,292-Fiore et al. issued Jan. 23, 1968, both of which are assigned to the assignee of the present invention. Such a screen differs from the screen of a conventional shadow mask color tube in two importantrespects. In the first place, the phosphor dots which constitute the dot triads of the now familiar mosaic screen 'are smaller in size than the exciting electron beams, therefore, the dots are fully illurninatedin the operation of the tube. Secondly, since the dots are of reduced size they are separated from one another whereas in the conventional dot type shadow mask tube, the dots are in tangential contact with one another. A pigment or any suitable lightabsorbing material such as graphite, manganese dioxide, ceramic black or the likeis used to fill in the screen portions that intervene the reduced-size phosphor dots of the tube. In short, each phosphor dot is surrounded by black material and, therefore, the screen is referred to as a black-surround structure. Its brightness and contrast are enhanced, as explained in the 3,146,368 Fiore patent.

Screening of dot triad picture tubes, whether they be conventional or of the more desirable black-surround variety, is generally accomplished photographically in which a coating of photo-sensitive resist applied tothe screen is exposed to actinic energy directed through the I openings of the shadow mask. In the simplest arrangement the photosensitivecoating also includes, as an ingredient, one of the necessary three phosphors and the exposure develops latent images of the dots of that phosphor distributed throughout the image area. Where sensitized polyvinyl alcohol is used as a photoresist, as is generally the case, simply washing the screenwith water removes the unexposed portions of the coating leaving developed dots of the particular phosphor in process.

Since they have been formed by exposure through the ice shadow mask from a light source positioned to simulate the electron beam of the tube that is to excite the phosphor being deposited, the phosphor dots are accurately located with respect to the shadow mask which is, of course, necessary for faithful color image reproduction.

It is quite simple by adjusting the intensity of the light source and the exposure interval to have the exposed portions of the photosensitive layer larger in size than the apertures of the mask. This simply entails utilization of the Well-known penumbra elfect and is totally satisfactory for preparing the conventional mosaic screen in which the phosphor dots are dimensioned to be in tangential contact. With the phosphor dots prepared in this fashion, however, the electron beams are smaller in diameter than the dots and their difference in size provides a guard band or tolerance for color and white field purity.

It is more difiicult to screen the black-surround tube in which the dimensioning of the dots and electron beams is reversed, that is to say, the dots are smaller than the electron beams. A number of proposals have been advanced in the past for processing such a screen, including the use of a photosensitive resist in which the exposure utilizes the penumbra effect and the exposure time is shortened so that the latent image of the phosphor dot is smaller than the aperture of the mask. In theory, this is an acceptable solution to the screening problem, but it is not a production-worthy process because it imposes severe tolerances if uniformity of dot size and shape is to be attained.

A much more attractive solution is referred to as etch back or re-etch. In that case, the mask is initially formed with holes of the size required for screening phosphor dots of a desired dimension and after screening has been accomplished, the mask is etched a second time or reetched in order to enlarge the holes so that the electron beams, which are themselves dimensioned by the size of the mask openings, are larger than the phosphor dots in a required amount to provide a guard band for color purity. This process has proved very production worthy and, in fact, is in commercial use.

The present invention proposes yet another and attractive screening process in which it is not necessary to rework the shadow mask. It permits forming that mask in the way it is prepared for conventional, as distinguished from black-surround, color tubes with apertures of a desired final size.

Accordingly, it is an object of the invention to provide an improved method of screening a black-surround tricolor cathode-ray tube.

It is a specific object of the invention to provide a novel method of forming the mosaic screen of a shadow mask color tube utilizing dot shaped phosphor deposits.

SUMMARY OF THE INVENTION The invention, as stated above, pertains to the screening of a tri-color cathode-ray tube having phosphor deposits arranged to define a series of dot triads distributed over the screen area and further having a shadow mask comprised of a field of apertures individually aligned with an assigned one of the dot triads. The method of the invention comprises the following steps: A layer of a photosensitive resist which becomes insoluble in response to exposure to actinic energy is applied over the entire screen. The layer is then exposed through the shadow mask to actinic energy from a source located at each of three exposure positions displaced a predetermined amount and in a first direction from three reference exposure positions which simulate the deflection centers of three electron beams which excite the triads in the operation of the tube. Following the exposure, the screen is treated with a solvent for the resist to remove the unexposed portions of the resist layer. Thereafter, a coating of light-absorbing material is deposited on those portions of the screen from which unexposed resist has been removed. After the lightabsorbing material is in place, the residuum of the resist layer is stripped. This same sequence of processing steps is then repeated at least one more time and in each sequence the screen is again exposed from three exposure positions displaced a predetermined amount and in a different direction from the reference exposure positions. The amounts and directions of displacement of the exposure positions in the several sequences are adjusted relative to one another to have the light-absorbing material deposited on the screen in those process sequences to define a coating pattern with a multiplicity of openings which expose elemental areas of the screen that are to receive phosphor materials to constitute the dot triads.

In its simplest embodiment the sequence of steps leading to the deposit of a light-absorbing matrix over the screen area is repeated twice. Roughly speaking, the first sequence of steps establishes one segment of the matrix having approximately semi-circular cutouts at the screen locations where phosphor dots are to be deposited. The second sequence of process steps forms the companion or complementary part of the matrix which also has approximately semi-circular cutouts and which, in conjunction with those of the first matrix segment, define recesses or openings to receive phosphor materials to constitute the phosphor triads.

The invention is broader in concept and is not confined to the use of two sequences of the process steps. For example, three such sequences may be utilized, as explained hereafter, although this does have the burden of additional processing over and above that of the two sequence embodiment described in the preceding paragraph.

BRIEF DESCRIPTION OF THE DRAWING The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings, in the several figures of which like reference numerals identify like elements, and in which:

FIG. 1 represents a fragmentary portion of the faceplate or screen structure of a color tube and is used in explaining an established prior art screening process;

FIGS. 2A and 2B are utilized in explaining the two sequence embodiment of the present invention with particular reference to a single phosphor dot;

FIGS. 3A and 3B relate to the same embodiment but on a single triad basis, while FIGS. 4A, 4B and 4C concern the same embodiment on a multi-triad basis;

FIG. 5A is a sketch used in discussing elliptical distortion and FIG. 5B concerns apparatus employed to minimize such distortion;

FIGS. 6A and 6B concern a three sequence embodiment of the invention; and

FIGS. 7 and 8 have to do with modifications of the three sequence embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Tri-color picture tubes have a conically shaped principal envelope section and a faceplate section, the latter being essentially a flanged dish having as a principal surface a spherical section which serves as the screen or image area of the tube. It may be round or rectangular but, in any event, is dimensioned and configured for uniting with the conical envelope section as by frit sealing. The initial separation of these envelope sections is a convenience in screening.

While the faceplate may have different configurations,

screening process is to establish a multiplicity of phosphor dot triads over the image area with each such triad having a dot of green, a dot of blue and a dot of red phosphor. These phosphor dots are dimensioned to be smaller than in a conventional dot triad type of color tube so that there is a portion of the screen separating or surrounding each dot into which pigment or light-absorbing material may be placed in themanner of a black surround tube described in the Fiore et al. patents. The screening process involves certain steps that are well known to the art, namely, the use of a photosensitive resist material that is rendered insoluble in a solvent upon exposure to actinic energy, whether that energy takes the form of impacting electrons of an electron beam or light of a selected waveit may also come in a variety of sizes; neither of these length. Alternatively, electrostatic screening, disclosed in Pat. 3,475,169 issued Oct. 28, 1969, may be employed but, for convenience, photosensitive resist screening will be described in detail. It is desirable in the commercial production of color tubes to make use of water based systems and, accordingly, the remainder of .this description will proceed on the premise that the photosensitive resist material is a water based solution of polyvinyl alcoholv (PVA) sensitized with ammonium dichromate which has the property that it becomes insoluble in water when exposed to ultraviolet light of sufficient intensity and for an exposure interval of sufiicient duration.

The ordinary use of this materialin screening a color tube will be explained briefly in relation to FIG. 1 which shows a segment 10 of the faceplate ofa color picture tube in process. It has on its concave innersurface a layer 11 of sensitized PVA. A conventional shadow mask 12 is shown in superposed relation with and spaced from faceplate 10 in the same positional relation that these elements have in the final tube assembly. This, of course, is assured by securing mask 12 to studs (not shown) provided on the inner wall of a flange which circumscribes the faceplate. The screen is normally provided with leaf springs (not shown) having apertures at the free ends for removably supporting the mask from the studs in desired positional relation to the screen. Specifically, the spatial relation is such that in the finished tube each aperture or hole of the shadow mask is in alignment with an assigned phosphor triad of the screen so that three electronbeams hav ing access to the screen solely through'the mask apertures impinge only upon their respectively assigned phosphor dots to accomplish color selection. Such a subassembly of faceplate and shadow mask is secured to a lighthouse or exposure chamber having a source of ultraviolet light positioned to simulate the electron beam of the tube which ly in process, usually as an ingredient of PVA layer'll. Exposure of that layer with ultravioletlight directed through the apertures of mask12 develops in layer 11 a latent image of each dot of the phosphor in process and these dots are developed simply by washing the screen with water after it has been exposed for an appropriate period of time. Of course, the developing step takes place after mask 12 has been removed from its position within the faceplate section. Washing the screen with water leaves dot-like deposits of insolubilized PVA including the phosphor in process. These. deposits shall have, been properly positioned relative to the openings of shadow mask 12 and are dot shaped as desired since the exposing light reaches the screen through the openings of the mask which are essentially circular. I

This is a well-known process of photographically prints ing a desired phosphor pattern on the screen and is quite generally used in the commercial production of tubes of the type under consideration. It may be utilized, for exam ple, in first developing the phosphor dot ,triadsby repeat ing this same general procedure for each of three color phosphors. The resulting phosphor triads are thereafter surrounded by a light-absorbing pigment as described in the 3,146,368 Fiore patent identified above. 'Alternatively and preferably, the light-absorbing material isifirst applie d to the screen in any of a variety of processes leading to the formation on the screen of a matrix that has holes into which respective phosphors may be deposited through the photographic printing technique that has justbeen described. Photographic printing of the phosphordeposits is compatible with the matrix forming process because -each uses the shadow mask as a pattern for locating the holes required in the matrix and for placing color pho sphors selectively in those holes. The present invention is most particularly concerned with the development of the matrix, preferably prior to the deposit of the phosphor materials and the rest of this discussion will be confined to the processing of such a matrix.

Broadly stated, the process follows in some measure the prior-art method described with respect to FIG. l but modified in two major respects to attain distinctively different results. The deviations from prior practices have to do principally with the exposure positionsand also the number of process sequences that are to be undertaken. With reference to FIG. 2A, the triangle G, B, and R represents a simulated plane of deflection of a color tube in process and, where the screening is in accordance with the prior art described above, further represents the positions of the light source for the three exposures that take place through the shadow mask. More specifically, the light source in each exposure is essentially circular, as shown, and is located at the appropriate apex of the triangle, depending upon whether the phosphor instantaneously in process is green, blue or red. Therefore, these three exposure positions G, B and R are considered reference exposure positions that simulate the deflection centers of the electron beams assigned to excite the green, blue and red phosphor elements of the dot triads of the screen, respectively. The dimension S is the displacement of each of these reference exposure positions measured from the center of the triangle which corresponds to the longitudinal tube axis and is usually of the order of 0.190".

The positions G, B and R' of FIG. 2A represent another set of exposure positions which are displaced from the reference set by a predetermined amount and in a given direction, specifically in a direction toward the center of the reference triangle, and represent an exposure condition for which the S distance has been changed; in particular, has been reduced by the amount AS. The designations G, B" and -R represent still another set of exposure positions, differing from the reference set in that now the S distance has been increased a particular amount in the opposite direction, that is to say, in a direction away from the center of the triangle. While the incremental change in S distance need not be the same, for both set's', it usually is at least approximately'the same and is in the range from 30 to 65 mils. For reasons to be explained hereafter, it is preferred to use a slit type of light source, as shown, rather than acircular source when exposing from the prime and double prime sets of positions. In practicing the present invention these sets of exposure positions are made use of in the following way.

First the screen, having been made chemically clean, receives a layer of a photosensitive resist which becomes insoluble in response to exposure to actinic energy. It has been assumed that a water-based material, such as sensitized PVA, is employed. The screen with the photoresist layer and having the shadow mask supported in position is installed in an exposure chamber to be exposed to actinic energy through the shadow mask. In the first exposure step the light source is located sequentially at each of three exposure positions, specifically positions G, B' and R of FIG. 2A, displaced as described a certain amount and in a first direction from the reference expo sure positions G, B and R. The exposed portions of the photoresist layer represent a latent image of a series of dot triads since the exposures are from each of the positions G, B and R so that treating the screen with the solvent, in particular, washing with water, develops those triads by removing the unexposed portions of the PVA layer from the screen. It should be stated in passing that the exposure does not occur simultaneously from each of the three exposure positions. Rather, the exposure is seriatim and this may be accomplished in known fashion by arranging the lighthouse so that the faceplate with its screen is indexable with respect to three exposure positions or, alternatively, arranging for the light source to be indexable as between three positions or may be exposed on separate tables for each color center. In each case, each such position establishes one of the exposure positions represented by one of the positions G, B and R of FIG. 2A.

A most significant change in the triad structure results from the displacement of the exposure positions from the reference positions G, B and R to the positions G, B and R. Specifically, where the exposure is from the reference positions all of the dots are uniformly spaced from one another throughout the screen area and usually the dimensions and exposure parameters are selected so that the dots are, additionally, in tangential contact. Exposing from the displaced exposure positions G, R and B may be characterized as bunching or grouping of the dots consistuting a single triad so that they overlap one another within the triad and, in end result, the dots are no longer equi-spaced throughout the screen area. Since they are no longer equi-spaced, the development step removes unexposed photosensitive resist from those portions of the screen which separate one dot triad from another. The next step in practicing the invention is depositing on those portions of the screen from which PVA has been removed a coating of light-absorbing material. A convenient way of applying the light-absorbing material is to make a slurry of colloidal graphite which is applied over the entire screen and becomes an overcoat for the developed triads of clear PVA. The graphite coating is then dried to fix it securely to those screen portions that surround the clear PVA dot triads. Thereafter, the residue of the PVA coating, namely, the PVA dot triads, is stripped from the screen by a chemical stripper, such as hydrogen peroxide. As the PVA is removed, the portion of the graphite coating which overlays the PVA triads is likewise removed so that the screen at this stage of the process has received one component of the matrix with ideally semicircular recesses defining a portion of each screen area that is assigned to receive a phosphor material.

To complete the matrix the same sequence of steps is repeated, including the application of the PVA layer over the entire screen, the exposing of that layer to ultraviolet from each of three exposure positions, the Washing of the screen with water, the deposition of light-absorbing mate-' rial and finally the stripping of the PVA triads without disturbing the portion of the matrix formed in the preceding sequence of process steps. This simply requires that the binder of the graphite be immune from attack by hydrogen peroxide which is the case with Aquadag for example. In this second sequence of steps, however, the exposure is from the second set of exposure positions G", B and R" likewise displaced from the reference set of exposure positions G, B and R but in a direction opposite from the displacement of the set of exposure positions G, B and R. While exposure from the inwardly displaced positions G, B and R caused bunching within the triads, exposure from the outwardly displaced positions G, B" and R has an opposite efiect and causes spreading of the dots outwardly from their own triad into overlapping relation with neighboring triads. This provides a space within each triad to receive light-absorbing material. The displacement of the exposure positions, both as to amount and direction, are adjusted in relation to one another in both the sequences of processing steps so that the lightabsorbing material deposited in the two sequences completes a matrix, defining a coating pattern with a multiplicity of approximately circular openings which expose those elemental areas of the screen that are to receive phosphor materials to constitute the dot'triads.

The formation of the matrix may perhaps be more clearly understood with reference to FIG. 2B, FIGS. 3A- 3B and FIGS. 4A-4C. Viewed on the basis of a single dot, and with particular reference to FIG. 2B, the circle 20 is a portion of the PVA layer that is exposed with the light source at an illustrative exposure position, for example, location G'. The exposed portion is represented ideally as circular and since the triad of which this dot is one element is separated from its neighbors and since graphite is applied in the area of separation, the arc 20a of dot 20 is surrounded by black-surround material which serves as a pair of the first major component of the matrix. On the other hand, circle 21 represents the portion of the PVA that is exposed in the next sequence of steps and from the counterpart position G" of the illustrative exposure position. Here, the dots are separated from one another within their own triads, in a fashion more clearly to be discussed with relation to FIG. 3B, and the arc 21a is surrounded by graphite that serves as a portion of the other or complementary component of the matrix.

The process is viewed on a triad as distinguished from a single dot basis in FIGS. 3A and 3B. The exposures from the set of positions G, B and R produces the bunched triad of clear 'PVA dots 22a, 22b and 220. The triangle simply designates the set of exposure positions from which this triad eventuates. It is clear that the graphite 22d surrounds a segment of the outer periphery of each PVA dot of the triad. This is because of the grouping which is manifest by the overlapping of the dots of the triad as indicated by the crosshatching of FIG. 3A.

The three exposures from the other set of positions G", B" and R" establish a degrouped triad as shown in FIG. 33 wherein the dots 23a, 23b and 23c are spaced from one another but overlap dots in the neighboring triads, again as indicated by the crosshatching. Also the triangle G", B" and R" denotes the relevant set of exposure positions. In this case the application of pigment in the sequence of process steps gives rise to the matrix portion 23d which surrounds segments of the inner peripheries of the dots of the triad Whereas the first application of graphite surrounded outer peripheral segments of the dots of the triad.

Viewed on an overall screen basis, although it is practical only to show a segment of the screen in FIGS. 4A- 4C, the first sequence of steps utilizing the set of exposure positions G, B and R establishes the first matrix component 24A of FIG. 4A. The second sequence of process steps utilizing exposure positions G", B" and R" of FIG. 4B produces the complementary component 24B of the matrix. Their total result is that of FIG. 4C, showing a matrix which has recesses for receiving phosphor materials to form the dot triads of the screen. One set of recesses is designated by the triangle g, b and r having apices centered in the holes of the matrix to receive phosphor.

The representations of FIGS. 24 are, as indicated, idealized in that they represent the exposed areas of the PVA layer to be circular when, in fact, there may be a tendency to elliptical distortion as represented, somewhat exaggerated for emphasis, in FIG. A. In practicing the described process of forming a black-surround matrix for a color tube and utilizing the usual ultraviolet light source with collimator having a circular tip a situation similar to that of FIG. 5A may be experienced. It is found that the recesses formed in the matrix tend to have an elliptical configuration similar to that designated 25. Where this occurs the relation of the electron beam to phosphor material deposited in the elliptical recess of the matrix is indicated by the superposition of pattern 26 representing the beam over pattern 25. This does not lead to the desired dimensional relation which by preference finds the beam larger in diameter than the phosphor dots in order to attain both full illumination of the dots and a 8 tolerance or guard band represented by the amount by which the beam diameter exceeds the dot diameter.

The x-dimension or major axis of the ellipse 25 may be controlled within limits by controlling the light pattern used for exposure while the y-dimension or minor axis of pattern 25 may be controlled by the amount of displacement of the sets of exposure positions G, B and R' and G", B" and R" with respect to the reference set G, B and R. It is apparent in FIG. 5A that control of the x-dimension is most desirable in order to more nearly approximate a dot shaped phosphor deposit and this may be accomplished by arranging the light source to project a linear light pattern which has a length dimension much greater than its width. That result may be achieved, for example, by suitably shaping and/or masking the collimator tip of the conventional lighthouse or, as shown in FIG. 58, by arranging a mask 2 having a slit 27a over the mercury lamp 28 usually employed as the ultraviolet light source in exposure chambers. A slot having a dimension of 30 x mils has been used effectively in ex:- posure processes where the AS dimension is in the range of 30-35 mils. Mask 27 should be arranged so that the linear light pattern is directed toward the tube axis, which is the center of the pattern of exposure positions. Of course, the narrower the slot the greater is the reduction in x-dimension but a compromise is necessary because there must be a certain minimum quantity of light in the exposure to effect the desired change in solubility of the PVA material. It has been determined, however, that dimensioning the light source as described in conjunction with adjustment of the displacement of the exposure positions relative to the reference exposure positions permits a very close approximation of circular recesses in the matrix which is necessary so that the effective phosphor deposit is also of dot shape.

The preferred practice of the invention features two sequences of process steps from exposure positions in the two sequences which are displaced in opposite directions from the reference exposure positions as described. The invention, however, is not limited in the number of process sequences that may be utilized. A three sequence embodiment is certainly practicable as indicated in FIG. 6A. The legend 6", B' and R" denote a third set of exposure positions. Accordingly, for each reference exposure position, for example, position G, there are three displaced exposure positions G, G" and 6'" each used in one of the three sequences, respectively. These exposure positions are displaced equal amounts from the related reference position G and define anrequilateral triangle which has an apex directed toward the center of'the reference triangle G, B and -R. As indicated in FIG. 7, however, the actual exposure positions may be modified so that the apex of the triangle which they define points outwardly from the center of reference. tri-.- angle G, B and R. Referring now to FIG. 6B in the first sequence of process steps circular portion 30 is exposed and the segment 30a thereof is enclosed with black-surround material in the same sequence of process steps. In the next sequence a portion 31 of the PVA layer is exposed and its segment 31a is enclosed with black-surround material in that same sequence. The final sequence causes portion 32 to be exposed and its segment 32a to be enclosed by black material. In this fashion the area of FIG. 6B enclosed by segments 30a, 31a and 32a is the matrix recess for receiving phosphor material.

As shown in FIG. 8, one may arrange the light source to have, in efiect, three tips 33a, 33b and 33c that would be energized individually in each of three .process sequences to obtain exposures similar to those described in connection with FIG. 6B. i

Referring once more to FIG. 4C, it will be seen that the holes or recesses of the light-absorbing matrix are substantially equi-spaced over the screen and they are smaller in size than the apertures of the mask. As a consequence, the phosphor dot areas effective in image reproduction are likewise smaller in size than the mask apertures and, therefore, smaller than the diameter of the exciting electron beams. This, of course, is the preferred arrangement which provides full illumination of the useful portion of each phosphor dot plus a purity guard band.

In mentioning the effective or useful area of the phosphor deposit, reference is had to the fact that in applying the phosphor material through the photographic technique described in connection with FIG. 1, the phosphor will close the assigned holes of the light-absorbing matrix and, usually, will cover at least some of the contiguous portions of the matrix. However, the matrix is substantially opaque and only the phosphor filling the holes of the matrix contributes to image reproduction. Accordingly, the effective phosphor deposit is smaller in size than the electron beams.

It should further be noted that the exposure time involved in forming the matrix is quite short because clear PVA is being processed. Moreover, the curve relating diameter of the insolubilized portions of the PVA to the intensity-time product of the exposure has a knee and operation with the subject invention is above the knee so that exposure times are not critical to obtaining a matrix with uniform and reproducible dimensions.

Experience with the two-sequence embodiment establishes that a black-surround screen with properly dimensioned and shaped phosphor dots is readily attainable.

While particular embodiments of the invention have been shown and described, modifications may be made, and it is intended in the appended claims to cover all such modifications as may fall Within the true spirit and scope of the invention.

I claim:

1. In the screening of a tri-color cathoderay tube having phosphor deposits arranged to define a series of dot triads distributed over the screen area of the faceplate of said tube and further having a shadow mask comprised of a field of apertures individaully aligned with an assigned one of said triads, the method which comprises the following steps:

(a) applying over said screen area of said faceplate a layer of a photosensitive resist which becomes insoluble in response to exposure to actinic energy;

(b) exposing said layer through said shadow mask to actinic energy from a source located at each of three exposure positions displaced a predetermined amount and in a first direction from three reference exposure positions which simulate the deflection centers of three electron beams which excite said triads in the operation of said tube to form on said layer a latent image of said field of apertures;

(c) treating said screen area with a solvent for said resist to remove the unexposed portions of said layer to develop a pattern of resist having a configuration corresponding to said field of apertures;

(d) depositing on said screen area a coating of lightabsorbing material;

(e) then stripping the exposed resist pattern from said screen area to expose portions of said faceplate;

(f) repeating the sequence of process steps -(a) through (e) at least one more time and in such sequence exposing from three exposure positions displaced a predetermined amount in a different direction from said reference exposure positions and adjusting the amounts and directions of the displacements of said exposure positions in said sequences relative to one another to have the light-absorbing material deposited in said sequences define a coating pattern with a multiplicity of openings which expose said faceplate and form those elemental areas of said screen area that are to receive phosphor materials to constitute said dot triads.

2. The method of screening in accordance with claim 1 in which said sequence of process steps is performed twice with the exposure positions of said source of actinic energy for the one sequence displaced inwardly of said reference exposure positions while the exposure positions of said source of actinic energy for the other sequence are displaced outwardly of said reference exposure positions.

3. The method of screening in accordance with claim 2 in which said reference exposure positions define a triangle and the inwardly displacement of said exposure positions for one sequence is in the direction of the center of said triangle while the outward displacement of said exposure positions for said other sequence is along the same path but in a direction opposite that of said inward displacement.

4. The method of screening in accordance with claim 3 in which said source of actinic energy comprises a rectangular light source for projecting approximately a linear light pattern having a length dimension greater than its width dimension.

5. The method of screening in accordance with claim 4 in which said light source, at each exposure position, is oriented so that the major axis of said light source is directed toward the center of the triangle defined by said reference exposure positions.

6. The method of screening in accordance with claim 4 in which the length of said light source and the amount of said displacements of said exposure positions in said one and said other sequence of process steps are selected to control the vertical and horizontal dimensions, respectively, of said openings in said coating pattern on said screen area so as to provide said openings with an essentially circular configuration.

7. The method of screening in accordance with claim 3 in which the exposure positions in said one and said other sequence are displaced approximately equal amounts from said reference positions.

8. The method of screening in accordance with claim 1 in which said sequence of steps is performed three times with the locations of said exposure positions in the sequences defining a triangular pattern about each of said three reference positions.

9. The method of screening in accordance with claim 8 in which the triagular pattern about each of said three reference positions is substantially equilateral with an apex facing in the direction of the center of the triangle defined by said three reference positions.

10. The method of screening in accordance with claim 8 in which the triangular pattern about each of said three reference positions is substantially equilateral with an apex facing outwardly from the center of the triangle defined by said three reference positions.

References Cited UNITED STATES PATENTS 2,733,366 1/1956 Grimm et al. 9636.1 3,146,368 8/1964 Fiore et al 9636.1 3,146,369 8/ 1964 Kaplan 31392 3,222,172 12/1965 Giuffrida 96-361 3,558,310 1/1971 Mayaud 96--36.1 3,573,909 4/1971 OFallon 96-.36.1 3,632,339 1/1972 Khan 9636.1

NORMAN G. TORCHIN, Primary Examiner 0 M. F. KELLEY, Assistant Examiner 

